A massive demonic invasion has overwhelmed the Union Aerospace Corporation's (UAC) Mars Research Facility, leaving only chaos and horror in its wake. As one of only a few survivors, you must fight your way to hell and back against a horde of evil monsters. The path is dark and dangerous, but you'll have an array of weapons–including a pistol, a chainsaw, grenades, and more–to use for protection.Go to: http://store.steampowered.com/ and download STEAM client
Click “Install Steam” (from the upper right corner)
Install and start application, login with your Account name and Password (create one if you don’t have).
Please follow these instructions to activate a new retail purchase on Steam:
Launch Steam and log into your Steam account.
Click the Games Menu.
Choose Activate a Product on Steam…
Follow the onscreen instructions to complete the process.
After successful code verification go to the “MY GAMES” tab and start downloading.